1. Field of the Invention
The present invention relates to a method and apparatus of blow molding a preform which has been previously formed by injection molding or extruding. Particularly, the present invention concerns a blow molding technique which utilizes molds arranged in two rows. 2. Description of the Related Art
In order to improve throughput, it is customarily performed in the blow molding technique that a plurality of hollow containers are simultaneously formed through one cycle including a series of steps such as injection molding step, blow molding step, ejecting step and other steps. One of the prior art systems for performing such a cycle comprises two rows of supporting plates for neck molds and two rows of blowing molds as described in Japanese Patent Publication No. 18847/1989. FIG. 9 is a cross-sectional view showing such a blow molding system which comprises an injection molding stage 10 for forming a preform and a blow molding stage 12 for forming a final product 22 from the preform. The system also comprises two rows of supporting plates 16 at each stage, each row of which are arranged spaced away from one another by a pitch P. Each of the supporting plates 16 holds one or more than two neck molds 14. The preform injection molding stage 10 includes two rows of injection-cavity molds 18 arranged opposed to the corresponding row of supporting plates 16 and spaced away from one another by the pitch P while the blow molding stage 12 includes two rows of blow-cavity molds 20 arranged opposed to the corresponding row of supporting plates 16 and spaced away from one another by the same pitch.
However, the pitch P between each row of supporting plates 16 must be selected to be relatively wide. This causes the entire system to increase in dimension because the pitch P must be determined to be compatible with the opening motion in the blow molding stage 12. More particularly, each of the blow-cavity molds 20 comprises a pair of mold halves 20a which are opened when a hollow product is to be removed out of the mold. Therefore, the magnitude of the pitch P is required to be equal to the total thickness of two inside mold halves 20a plus the movement of the blow mold halves 20a on being opened. Simultaneously, each of the mold halves 20a must have a sufficient thickness to resist a given internal pressure (blowing pressure) without flexure. Thus, such a thickness in each mold half 20a will be added into the thickness of a backing plate (not shown) for supporting that mold half. This will cause the pitch P to increase.
For such a reason, the entire molding system cannot but increase in size and also occupy a larger space. Such a problem is exaggerated when it is wanted to form hollow containers having an increased diameter. Two rows of injection molds or temperature regulating sections can be arranged at most in alignment with the rows of blowing molds. In addition, this results in increase of conveying area.